1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the optical scanning apparatus. In particular, the present invention is suitable for an image forming apparatus using a plurality of light emitting elements (light emitting parts) as a light source, so as to achieve high speed and high density recording and using an electrophotography process, such as a laser beam printer, a digital copying machine, or a multi-function printer.
2. Description of the Related Art
Conventionally, a multi-beam optical scanning apparatus employing a plurality of light emitting elements (light emitting parts) as a light source has been widely used in a laser beam printer, a digital copying machine, and the like. As the multi-beam optical scanning apparatus, one having such a configuration that causes light beams to enter in parallel a surface perpendicular to a rotation axis of a deflecting surface of a deflector has been known. In this type of multi-beam optical scanning apparatus, scanning line pitches in a sub-scanning direction of scanning lines drawn on the same surface to be scanned by the light beams from a plurality of light emitting elements (light emitting parts) are set to be uniform in the whole effective scanning area. With this configuration, a magnification in the sub-scanning direction of an imaging optical system between the deflecting surface of the deflector and the surface to be scanned is set to be uniform in the whole effective scanning area.
On the other hand, in recent years, a configuration that causes the light beams to enter the plane perpendicular to the rotation axis of the deflecting surface of the deflector in a direction with an oblique angle with respect to the plane within a sub-scanning section (hereinafter referred to as an “oblique incident optical system”) has often been used for scanning a plurality of surfaces to be scanned with a single deflector to downsize the apparatus. When a multi-beam light source is used in this oblique incident optical system, if the magnification in the sub-scanning direction of the imaging optical system is set to be uniform in the whole effective scanning area, a scanning line interval becomes non-uniform, causing an uneven interval. To cope with this problem, in Japanese Patent Application Laid-Open No. 2009-008896 (corresponding to US 2009/0002474), a method of achieving a uniform scanning line interval has been disclosed in which the magnification in the sub-scanning direction of the imaging optical system is set to be different between a scanning start side and a scanning end side.
Further, in Japanese Patent Application Laid-Open No. 2009-014953 (corresponding to US 2009/0009841), a technology for improving the performance has been proposed in which a sagittal line shape of an optical surface of the imaging optical system is formed in a non-arc shape, to meet a further demand for high density and high resolution recording. In Japanese Patent Application Laid-Open No. 2009-014953, both a field curvature correction in the sub-scanning direction and a reduction of the uneven scanning line interval caused by a shift of an image position in the sub-scanning direction due to a tangle of the deflecting surface are achieved by forming the sagittal line shape of the optical surface of the imaging optical system in a non-arc shape and varying an aspheric coefficient of the optical surface along a main scanning direction.
However, when the technology disclosed in Japanese Patent Application Laid-Open No. 2009-014953 is applied to a multi-beam optical scanning apparatus having the oblique incident optical system, the following problem occurs. That is, because the light beams respectively pass through different positions separated from each other in the sub-scanning direction on the optical surface having the non-arc sagittal line shape, barycentric positions of a spot intensity distribution are shifted from each other. Therefore, as described in Japanese Patent Application Laid-Open No. 2009-008896, even when the geometric optics interval of the scanning line based on principal ray arrival positions of the light beams from the light emitting elements (light emitting parts) are set to be uniform on the surface to be scanned in the main scanning direction, a wave optics interval of the scanning line is not uniform in the main scanning direction due to a barycentric position shift of a spot intensity distribution.